fs/ext4: fix log2blksz un-initialized error, by cacaulating its value from blksz

The problem here is that uboot can't mount ext4 filesystem with
commit "50ce4c07" applied. We use hard-coded "SECTOR_SIZE"(512)
before this commit, now we introduce (block_dev_desc_t *)->log2blksz
to replace this macro. And after we calling do_ls()->fs_set_blk_dev(),
the variable log2blksz is not initialized, which it's not correct.

And this patch try to solve the problem by caculating the value of
log2blksz from variable blksz.